Cellulosic and lignocellulosic feedstocks and wastes, such as agricultural residues, wood, forestry wastes, sludge from paper manufacture, and municipal and industrial solid wastes, provide a potentially large renewable feedstock for the production of valuable products such as fuels and other chemicals. Cellulosic and lignocellulosic feedstocks and wastes, containing carbohydrate polymers comprising cellulose, hemicellulose, and lignin are generally treated by a variety of chemical, mechanical and enzymatic means to release primarily hexose and pentose sugars, which can then be converted to useful products with processes such as thermal, enzymatic, chemical, and/or biological treatments.
Impediments to release of sugars from cellulosic biomass include its protection by lignin and cellulose crystallinity. Pretreatment methods, including steam explosion, hot water, dilute acid, ammonia fiber explosion, alkaline hydrolysis, oxidative delignification and organosolv (Zhao et al. (2012); Biofuels, Bioproducts and Biorefining 6(5): 561-579), are often used to make the carbohydrate polymers of cellulosic and lignocellulosic materials more available for saccharification. Mechanical size reduction is often used in combination with chemical treatments to make cellulosic biomass amenable to saccharification, primarily to create more surface area of the biomass to speed up reactions. Costs of chemicals, chemical recovery, energy inputs, and capital equipment make many pretreatment methods not amenable to commercial production.
Using mechanical treatment alone prior to saccharification, where only size reduction takes place, does not eliminate the aforementioned issues that hinder sugars release: protection by lignin and cellulose crystallinity. However, mechanical treatment alone has been used to affect the cellulose crystallinity thereby increasing release of sugars, but typically requires high energy input. Hick et. al (Green Chemistry (2010) 12(3): 468-474) experimentally show that at small scale (1 gram batches) shaker mills, wherein spherical media are shaken with material inside of a container, can sufficiently destructure cellulose for full enzymatic release of glucose, but at energies upwards of ˜500 kJ/g, which is ˜25× the combustion energy of the biomass (˜17-22 kJ/g) (McKendry Bioresource Technology (2002) 83(1) p 37-46). Larger scale stirred ball mills (1 kg) can achieve a 5× reduction in specific energy to ˜100 kJ/kg. Simulated results of larger scale attrition milling (stirred ball milling) (100 kg) predict energies >15 kJ/g, or at least 75% the energy of the biomass.
In an attempt to create a more energy efficient mechanical process for the release of sugars, Takahashi et al. (Japan Society of Mechanical Engineers collected papers; Note No. 2011-JBR-0845; 78 No. 788 (2012-4)) use a gear type grinding-media mill. The biomass was milled in batch inside a cylindrical chamber that vibrates around a central axis. Circular gears are placed inside the cylindrical chamber as free flowing media, and are allowed to freely move around inside the container as the container vibrates, causing a grinding action against the wall. Biomass particle size diameter was reduced on average from 55 μm to 20 μm in 20 minutes, and the saccharification efficiency of holocellulose (combined cellulose and hemicellulose) reached around 70% at pulverizing time of 60 minutes. The specific energies required to achieve the sugar yield above still required ˜40-103 kJ/g. In a similar report by Mori et al. (Japan Society of Mechanical Engineers collected papers; Note No. 2011-JBN-0582; 78 No. 787 (2012-3)), biomass was continuously processed in a continuous high-impact pulverizing vibration mill without the use of gears, but using smooth surfaced rings. The saccharification efficiency of holocellulose reached ˜50% at pulverizing time of 60 minutes for a continuous pulverizing process and about 65% for a batch process run for 60 minutes. These lower sugars yields still required a specific energy of ˜8-10 kJ/g, which is ˜40% of the combustible energy of the biomass. Stresses on the material reach almost 20,000 psi (137.9 MPa), and the force applied is <5,000 N.
There remains a need for lignocellulosic biomass pretreatment processes that use lower specific energies to create destructured cellulosic biomass from which sugars can be obtained. Such processes can reduce energy costs and increase investment productivity relative to alternative strategies.